Temperature compensated klystrons



Sept. 9 1969 t. E. WARD 3,466,495

TEMPERATURE COMPENSATED xm's'rnons Filed Feb. 27, 1967 2 Sheets-Sheet 1 PRIOR ART L l4 I9 NVENTOR.

TIS EJARD Sept. 9, 1969 c. E. WARD TEMPERATURE COMPENSATED KLYSTRONS 2 Sheets-Sheet 2 Filed Feb. 27, 1967 FIG.?

United States Patent 3,466,495 TEMPERATURE COMPENSATED KLYSTRONS Curtis E. Ward, Los Altos, Califl, assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed Feb. 27, 1967, Ser. No. 618,964 Int. Cl. H01j 25/22 US. Cl. 3155.23 7 Claims ABSTRACT OF THE DISCLOSURE This invention relates in general to high frequency electron discharge devices of the klystron type and more particularly to novel temperature compensation mechanisms for such klystrons.

The theoretical and practical design techniques for klystrons of the coupled cavity, leaky wall tuner, reflex and external cavity types utilizing iris coupling means between a resonant cavity and another resonant or nonresonant region of the klystron are well known. It is also well known that stabilization of the operating frequency of such devices during normal operation and in operation under variable environmental conditions is a complex problem which has found different types of solutions. For example, one prior art approach to the problem of differential thermal expansion occurring in use in a conventional klystron wherein the cavity dimensions increase under thermal stress as well as the gap dimensions is found in US. Patent No. 3,222,565 wherein a temperature compensation mechanism is employed in the collector region of the tube to control gap spacing.

The present invention provides a more direct solution to alleviate frequency drift due to thermal changes in usage of klystron tubes in a manner novel in the art. The technique basically involves utilization of a differential thermal expansion mechanism in the iris coupling portion of the tube regardless of where the iris may be located. For example, in a simple external cavity tuned reflex klystron the coupling iris between the internal and external cavities can be shown to perform a very critical function with regard to the operating frequency and its dimensions are highly critical.

The present invention teaches several ways of utilizing differential thermal expansion mechanisms within the iris itself to control the coupling between the internal and external cavities of such klystrons so as to restrict any frequency deviations due to iris dimensional changes to a comparatively minimal level. Another type of klystron oscillator utilizing a leaky wall tuning mechanism as a below cutoff waveguide tuner coupled to one or more of the operating cavities of the klystron can also be beneficially enhanced with regard to the iris coupling mechanism between the waveguide and the tube cavity by utilizing the differential thermal expansion iris compensation mechanism of the present invention as will be more detailed hereinafter.

The alleviation, by the teachings of the present invention, of the variation in coupling due to dimensional changes in the coupling iris which interconnects coupled cavities or any two electromagnetically coupled systems in klystron devices by utilization of the techniques of the present invention will obviously render the heretofore rather complex mechanisms for maintaining frequency stability in klystron oscillators in particular as shown in US. Patent No. 3,222,565 a more simple problem since a major cause of such frequency deviations is shown by the present invention to be the iris itself. Thus other solutions which have heretofore been employed for maintaining frequency stability in klystrons such as utilization of particular header angles, bowed headers, and thermal compensation by gap spacing techniques can be accomplished in a much more simple and less complex manner.

In order to show the rather drastic effect which the dimensions of an iris between a coupled cavity system have, the following simple analysis should serve as a further clarification thereof. A simple oscillator circuit can be represented as an LC circuit wherein L C is the coupled circuit portion, and m. is simply a the coupling factor or mutual coupling therebetween. The operating frequency of this circuit can be shown to be proportional in the following manner:

Utilizing the techniques of klystron construction, it is self-evident that m is achieved in most cases through utilization of a coupling iris between the coupled systems. The following interesting relationship between the dimensions of the coupling iris and m, the coupling factor or mutual coupling can be established.

mad for a normal round iris, and maW h for a rectangular iris, where d is diameter Wis width h is height Upon examination of the above relationships in conjunction with the respective mutual coupling on the operating frequency of the coupled system, it is self-evident that the dimensional changes of the iris can have profound effects on the operating frequency of the device itself. As a matter of fact, frequency deviations of 200 kilocycles per degree centigrade variation for operating frequencies in the gigacycle range is not at all uncommon unless appropriate measures are taken to prevent such frequency deviation by other complex mechanisms as explained previously.

The present invention as will be detailed hereinafter will provide another simple approach which will help to alleviate such frequency stability problems in klystrons.

It is therefore an object of the present invention to provide a high frequency electron discharge device of the klystron type with novel differential thermal expansion iris temperature compensation means.

A feature of the present invention is the provision of a high frequency electron discharge device of the klystron type which includes an iris coupling aperture with a diiferential thermal expansion iris temperature compensation mechanism for rendering the coupling through the iris comparatively insensitive to temperature variations.

Another feature of the present invention is the provision of a high frequency electron discharge device of the klystron type which includes an iris coupling aperture with a metal insert having greater ductility and larger thermal co-efficient of expansion than the surrounding device body portion forming the internal defining wall portions of the iris.

These and other features of the present invention will become more clearly apparent upon a perusal of the following description taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a cross-sectional view partly in elevation of a high frequency electron discharge device of the klystron type and, more particularly a reflex klystron which incorporates the temperature compensated iris coupling mechanism of the present invention.

FIG. 2 is a fragmentary view of a typical uncompensated iris of the prior art taken along lines 22 of FIG. 1.

FIG. 3A is a fragmentary view showing a temperature compensated iris according to the teachings of the present invention.

FIG. 3B is a rectangular version of the circular iris of FIG. 3A.

FIG. 4 is a schematic view of an iris depicting the relative change in a ring compensator for explanatory purpose only.

FIG. 5A is a fragmentary cross-sectional view of a coupling iris according to the teachings of the present invention utilizing a slotted temperature compensation mechanism in a round or circular version.

FIG. 5B is a rectangular version of the temperature compensating mechanism of FIG. 5A.

FIG. 6 is a schematic view of a slotted temperature compensation insert for purposes of explanation only.

FIG. 7 is a cross-sectional view partly in elevation of a reflex klystron incorporating a leaky wall type of tuning mechanism having a temperature compensated FIG. 8 is a cross-sectional view taken along lines 8-8 of FIG. 7 of the iris portion.

FIG. 9 is an illustrative graphical portrayal showing a relationship between the operating frequency of a klystron and the resonant frequency of an iris coupling mechanism between the internal and external cavity arragement showing the rather adverse effects that a variation of the coupling through the iris may have upon the operating frequency of the tube.

Turning now to FIG. 1 there is depicted a reflex klystron having an electron gun and cathode assembly 13 disposed at the upstream end portion thereof and a reflector mechanism 14 disposed at the downstream end portion thereof. A resonant cavity 15 is bounded by a pair of header members 16, 17 disposed Within steel body 18. An external cavity 21 is coupled to the internal cavity 15 as shown. Energy is extracted from the cavity 15 via iris 19 and dielectric wave permeable window 20 as of alumina which is bonded to the internal walls of cavity 21. The reflex klystron of FIG. '1 is tunable via tuning screw 22 in a conventional manner. Since this type of reflex klystron is well known in the art, further description thereof is deemed unnecessary. For more details on such a tube see, for example, U.S. Patent No. 2,880,357 by D. L. Snow et al., US. Patent No. 2,815,467 by B. C. Gardner and US. Patent No. 2,789,250 by S. F. Varian et al., each of which is assigned to the same assignee as the present invention.

In any coupled cavity system such as shown in the klystron depicted in FIG. 1 the coupling iris 19 plays a critical function as set forth previously. Generally speaking, turning now to FIG. 2, the coupling iris will take the form of a simple circular, rectangular, oval or other type configuration as for example a simple circular bore 19 in the steel body 18 of the tube itself.

Turning now to FIG. 3A, a differential thermal expansion temperature compensation mechanism employing a circular insert 27 of for example copper dimensioned so as to have an internal diameter d equal to the uncompensated internal diameter of the iris of FIG. 2. By making the insert 27 of a ductile material relative to the material of the tube main body and of a material which has a higher thermal co-eflicient of expansion than the tube main body a simple differential thermal expansion mechanism for rendering the coupling of the iris relatively insensitive to normal temperature variation is achieved.

The basic mechanism by which both the circular embodiment utilizing the circular insert 27 and the rectangular embodiment of FIG. 3B utilizing the rectangular insert 28 accomplished the function of rendering iris coupling relatively less sensitive to thermal expansion problems can be stated simply as follows:

Referring to FIG. 4 and assuming that a copper ring is brazed within a steel body the effects of a C. change in temperature on coupling will be computed using typical values for purposes of illustration only.

5 =coefiicient of thermal expansion of steel 10X 10 inches/in./ C.

6 =coeflicient of thermal expansion of copper =16 1O inches/in./ C.

Now assume a plate of steel with no apertures but with a circle of diameter D scribed on the surface. At T the diameter of the scribed circle is D and at T =T +AT the diameter of the scribed circle will be let D :1.000 AT=100 C. 5 =10 10* inches/inch/ C.

thenD =l.0O0+100 10 10 1.000

If the scribed circle were an iris and coupling mtrD the new coupling factor will be 21.006 which is a significant change.

Now it is obvious that since the metal inside the scribed circle can be removed at either T or T with no effect, since with the mass of steel at a uniform temperature there will be no tensile or compressive stresses within the steel, we can apply the same reasoning and achieve the same results if we say that we start with a hole in the steel plate, the hole will become larger if the temperature of the steel plate is increased.

If however, we had subjected a copper disk to an increase in temperature corresponding to AT, the copper will increase in dimension more than the steel since its expansion coeflicient is greater than steel. Similarly, a copper ring will increase both its outside diameter and its inside diameter when heated and by greater amounts than would a steel ring of similar initial dimensions.

In the instance of FIG. 4, we have placed and secured by brazing a copper ring within a hole in a steel plate. As the assembly is heated, the hole in the steel plate will enlarge by an amount ATB D The copper will also attempt to enlarge, by an amount ATfi D but cannot do so because it cannot become larger than the hole in the steel. The excess expansion of the copper ring will result in a compressive force between the copper and the steel. Since steel is much stronger than pure copper, the copper ring will be squeezed by the steel in such measure that the compressive force is always limited to the relatively low compressive yield point of pure copper. We can assume, without large errors, that the residual force can be ignored and we can also assume that the copper ring has an appreciable thickness, such as to of d along the axis of the diameters such that the deformation under stress will be predominantly radial.

(a) The area of the ring =1r/4(D d when cool (b) Area of the ring healed but not restrained 4: (D +D AT5 )"(ch-l-thATJ Area of the ring heated and restrained. In this case the large diameter will be determined by the steel which has also been heated. The diameter of D in steel when heated is:

So that the area of the copper ring, heated and restrained, is

note the d is the same as in (a) above-we started with the requirement that d not change when the assembly was heated.

(d) Area heated and restrained=Area heated not restrained very closely.

Since we were interested in determining the ratio of D to d h d a., a, (f) Now to put in real numbers for examples:

Copper ring in steel plate:

6 16 in./in./ C. 6 =l0 10 in./in./ C.

Copper ring in molybdenum plate:

6 =16 10-- in./in./ C. 8 =5 10- in./in./ C.

n 1 Fa d 16-5 11 Obviously as the expansion coefficient of the plate approaches zero the ratio of D /d approaches 1less material is required in the ring to permit compensating d.

An analysis of rectangular, square, oblong, etc. devices would be done in the same manner on the basis of area changes as discussed above and need not be detailed further.

In other Words, since tubes typically operate in a range of temperature between around 0 C. to 200 C., one

can determine the 6 of the materials at C. and get good results. Since the above analysis depends only on area the approximate equation:

EN d

A simple example would involve a steel body or support frame and a copper insert having an iris diameter d and an external diameter 1.63d disposed in a steel body as shown in FIG. 3A with an iris thickness dimension 1 of /5 to A of d. This inter-relationship will produce a fairly constant iris diameter d under normal thermal variations experienced by klystrons. The steel body 18 expands increasing the dimensions of 1.63d and the copper insert 27 expands increasing its exterior dimensions within the limit of the increase in the steel body 1.63d dimension. The additional expansion of the copper insert over the expansion of the steel body reduces the expansion of the iris dimension d to a net change of zero. The insert 27 is brazed with any conventional brazing material, e.g., silver or copper-gold to the steel body. The ductility of the copper insert relative to the steel frame maintains the bond.

Obviously, one hundred percent compensation without any deviations whatsoever in the coupling is an extremely diflicult goal to obtain and would require a complex mathematical analysis of each and every parameter involved and then it would be diflicult physically to accomplish the desired results.

However, improvements over the uncompensated case as shown in FIG. 2 are achieved by utilizing the insert approach of FIGS. 3A and 3B.

Turning now to FIGS. 5A and 5B there is depicted circular and rectangular versions of ductile and slotted inserts of high temperature coeflicient expansion materials. The incorporation of the slots has been found to have advantageous benefits in cases where the iris thickness is made extremely thin. The slotted versions as shown in FIGS. 5A and 5B function in the same manner as those in FIGS. 3A and 3B by maintaining the iris interior dimensions substantially constant under thermal stress and eliminate any buckling problems which may otherwise occur in a very thin version of the iris differential thermal expansion compensation technique depicted in FIGS. 3A and 3B.

In FIG. 6 a schematic representation is shown for a single finger slotted case and is given as an aid to understanding the design approach for the slotted case. The analysis is independent of the fact that a pair of fingers are diametrically opposed to each other since the fingers may be analyzed independently of each other due to the slots which permit the insert to expand sideways.

In the case of FIGS. 5A and 5B, showing parts with slots, the effect of the expansions has different mathematics. The slots allow the fingers to expand sideways, so that we must calculate for linear expansion rather than relating through area. Consider a hole with only one copper finger. In this case we want to keep the gap at the end of the finger as constant. This gap is L -L 7 When heated L becomes (L +AT6 L When heated L becomes (L +AT L If gap is equal, hot or cold:

For example: if finger is copper and block is steel If finger is copper and block is molybdenum & 16 10' in./in./C L 5X iI1./in./C

This solution applies with any number of fingers, as long as the slots are sufficient to prevent any finger from affecting dimensions of the next finger. Again 6 =5 and 6 :6 as above are used to generally define the slotted case regardless of the insert configuration.

And then where L dimension of aperture in direction of finger axis L =finger length 6 =coefficient of thermal expansion of insert 6 =c0effici6nt of thermal expansion of wall wherein 6 and 3 are determined at around 100 C. For squares and circles only one measurement need be made. For rectangular and oblong cases, two measurements should be made to optimize the design, namely, along the major and minor axes. The slots should extend over a substantial portion of the insert thickness so that the unslotted portion can be ignored in the computations. Obviously, other more precise calculations can be made if desired.

Turning now to FIG. 7 there is depicted a reflex klystron of conventional design incorporating a leaky wall or cutoff waveguide tuning mechanism as depicted in more detail in US. Patent application Ser. No. 500,970 by Charles D. Mack, filed Oct. 22, 1965 and assigned to the same assignee as the present invention. This leaky wall tuner mechanism is adequately described therein and will not be repeated herein. In brief, the reflex klystron depicted in FIG. 7 includes a re-entrant cavity resonator 50 representative of an interaction vehicle for electromagnetic fields and an electron beam traversing therethrough to provide cumulative interaction in a manner well known in the art.

The reflex klystron includes an electron gun 51 of any conventional type such as a Pierce type gun focusing electrode 52 and cathode assembly 53 disposed at the upstream end portion thereof for producing and directing an electron beam through the re-entrant cavity 50 and to the reflector region at the downstream end portion thereof where a suit-able negative voltage on the reflector electrode 55 repels the electrons in a conventional manner back through the interaction gap to generate a microwave signal. A reflector electrode 55 is supported by any suitable conductive rod member 56 disposed along the beam axis 57 and supported by means of any suitable dielectric disc 58 vacuum sealed across the tube main body 59 at the downstream end portion thereof. Similarly, the gun 51 is supported by means of dielectric disc 60 vacuum sealed across the main body 59 at the upstream end portion as shown. The cavity resonator is defined by a pair of axially spaced header members 61, 62 to form cavity end walls having axially aligned apertures 63, 64 preferably provided with suitable grids 65, 66 for enhancing the beam coupling interaction. The tuner mechanism includes a hollow circular waveguide 70 of copper or the like, vacuum brazed in a conventional manner around the coupling aperture of the iris 71. A dielectric vacuum window 72 maintains vacuum integrity between the cutoff waveguide 73 and the resonator 50 and is brazed or the like to the side walls of the chamber as shown. Tuning screw 74 is utilized to control the H field penetration into guide 73 to control the operating frequency. Electromagnetic wave energy is extracted from the output waveguide and through dielectric window 81 vacuum sealed across iris 83.

As discussed in connection with the embodiment of FIG. 1 changes in the internal dimensions of an iris such as 83 have a pronounced effect upon the operating frequency of the tube. Similarly, changes in the internal dimensions of an iris such as 71 also have a pronounced eifect on the operating frequency of the tube. This embodiment is shown to illustrate the diverse regions where coupling irises can be utilized in a klystron and serves to illustrate the generic application of the temperature compensation techniques of the present invention. In particular, iris 71 is shown in FIG. 8 and is of oval or ellipsoidal shape rather than circular or rectangular. In the same manner as shown in FIGS. 3 through 6 any of the temperature compensated mechanisms depicted therein may be employed.

Turning now to FIG. 9 there is depicted an illustrative graphical portrayal of coupling vs. frequency for an internal-external coupled cavity klystron device. The klystron operating frequency is depicted at f and the coupling iris resonant frequency is generaly much higher such as at f The coupling between the two cavities denoted by Curve A regardless of where f is located should remain constant for stable operation. However, as discussed previously, variation of the coupling of the iris by increased inductance will lower 1; and thus increase the coupling at f and reduce the tube operating frequency unless compensated. Therefore by utilizing the appropriate techniques as discussed previously, it is possible to maintain the coupling through the iris substantially constant and thus obviate any variation in the operating frequency of the klystron due to iris dimensional changes.

Since many changes could be made in the above construction and many widely different embodiments could be made without departing from the scope thereof, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A high frequency electron discharge device of the klystron type incorporating electron gun means disposed at the one end portion thereof for generating and direct ing an electron beam along the beam axis of the device, a cavity resonator disposed about the beam axis, said cavity resonator including iris coupling means for coupling electromagnetic energy between said cavity resonator and another region of said device, said coupling iris including differential thermal expansion means incorporated therein for reducing the sensitivity of the coupling coeflicient of said iris to temperature variations of said device.

2. A high frequency electron discharge device of the klystron type including an iris coupling means disposed therein for coupling electromagnetic energy between a resonant cavity and another portion of said device, said iris including differential thermal expansion means for maintaining the coupling through said iris relatively constant over a range of device temperatures.

3. The device defined in claim 2 wherein said differential thermal expansion means is a round metal insert disposed within an aperture defined by a first metal member and defining the internal peripheral surface of said iris, said ring insert having a major outer dimension D; and a major inner dimension d taken along the same line are chosen such that where 6 =coefiicient of thermal expansion of the insert 6 =cefiicient of thermal expansion of the first metal member, wherein 6 and 6 are determined at around 100 C.

4. A high frequency electron discharge device of the klystron type such as defined in claim 2 wherein said differential thermal expansion means includes a metal insert which defines the internal iris dimensions, said insert disposed within and rigidly afiixed to a wall member of said device, said insert being made of a material having a higher coefficient of thermal expansion than said wall member said insert being more ductile than said wall member.

5. The klystron defined in claim 4 wherein said insert is provided with a plurality of slots extending from the internal peripheral defining portions thereof toward said wall member to thereby define a plurality of slotted regions along the internal peripheral portions of said iris. 6. The klystron defined in claim 5 wherein said slots define fingers therebetween and wherein a finger length L and an outer width dimension of the insert L as determined along the finger axis are related such that kin L 2 53 where 5 =coefficient of thermal expansion of the insert 6 =coefiicient of thermal expansion of the wall member wherein 6 and 6 are determined at around C.

7. A high frequency electron discharge device including a pair of coupled cavity resonators, said pair of coupled cavity resonators having iris coupling means for coupling electromagnetic energy therebetween, said iris coupling means including means for controlling the coupling through said iris coupling means as a function of temperature change of the iris, said means for controlling the coupling through said iris coupling means including at least two difierent metals forming a part of said iris coupling means, said one metal having a different thermal expansion coefiicient than said other metal, and said one metal having a difierent ductility than said other metal.

References Cited UNITED STATES PATENTS 2,815,467 12/1957 Gardner 315-521 2,967,973 1/1961 Vaccaro 315-3953 HERMAN KARL SAALBACH, Primary Examiner SAXFIELD CHATMON, JR., Assistant Examiner U.S. Cl. X.R. 

